Methods and apparatus for evaluating radiated performance of mimo wireless devices in three dimensions

ABSTRACT

According to some embodiments, a system is provided for simulating a cluster of reflections. The system includes an array of antenna elements distributed in space over a solid angle having an angular spread. The solid angle is substantially less than a full sphere and each antenna element has a spatial orientation. The system also includes a spatial environment simulator connected to the antenna elements and configured to apply one of excitations to the antenna elements and weights to signals from the antenna elements. The combination of the spatial distribution of the antenna elements, the orientation of the antenna elements, and the weighting applied by the variable path simulator simulates a near field arising from a cluster of reflections of a multipath environment.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to and claims priority to U.S. ProvisionalPatent Application Ser. No. 61/708,320, filed Oct. 1, 2012, entitledMETHODS AND APPARATUS FOR EVALUATING RADIATED PERFORMANCE OF MIMOWIRELESS DEVICES IN THREE DIMENSIONS, the entirety of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

n/a

FIELD OF THE INVENTION

The present invention relates to a method and system for evaluatingradiated performance of Multiple Input Multiple Output (MIMO) wirelessdevices in three dimensions.

BACKGROUND OF THE INVENTION

The advent of wireless technologies such as LTE and 802.11n, which takeadvantage of the multipath nature of radio propagation in the realworld, have led to the development of test methods for evaluating theradiated performance of these devices in a controlled simulatedenvironment. Current embodiments of these test systems generallyevaluate the performance in a statistically uniform, highly multipathenvironment (reverberation chamber) or in a uniform azimuthal boundaryarray capable of simulating a wide variety of environments through theapplication of an appropriate channel model, where the device is rotatedthrough the simulated environment to determine average performance.While the latter boundary array method can generally be expanded toprovide full spherical environment simulation, the implementations aregenerally cost prohibitive. In addition, both methods generally provideaverage performance metrics without significant detail on theorientation specific impact of the device performance.

SUMMARY OF THE INVENTION

Embodiments advantageously provide systems and methods for evaluatingthe performance of wireless devices in a simulated multipath RFenvironment. In one embodiment, a plurality of antennas are arranged ina constellation distributed over a solid angle, such as on thetwo-dimensional surface of a sphere, to simultaneously transmit orreceive signals simulating a single cluster of reflections in a real 3-Denvironment. A variety of different mechanisms may then be used torotate the cluster relative to the device under test (DUT) to map athree dimensional picture of the device's performance in the presence ofa faded multipath channel.

According to some embodiments, a system is provided for simulatingradiation from a cluster of reflections. The system includes an array ofantenna elements distributed in space over a solid angle having anangular spread. The solid angle covers substantially less than a fullsphere and each antenna element has a spatial orientation. The systemalso includes a variable path simulator in communication with theantenna elements. The combination of the spatial distribution of theantenna elements, the orientation of the antenna elements, and weightingapplied by the variable path simulator cause simulation of a near fieldenvironment arising from a cluster of reflections of a multipathenvironment.

According to some embodiments, a method is provided for simulatingradiation from a cluster of reflections. The method includes providing aspatial distribution of antenna elements over a solid angle having anangular spread, where the solid angle covers substantially less than afull sphere. The method includes connecting each antenna element to avariable path simulator. The variable path simulator is configured tocommunicate with the antenna elements. The method also includesadjusting at least one of the spatial distribution, orientations, andweighting applied by the variable path simulator to simulate a nearfield arising from a cluster of reflections.

According to some embodiments, an anechoic test chamber is provided totest a response of an RF device to a cluster of reflections of amultipath environment. The anechoic test chamber includes an enclosedroom and a plurality of antenna elements. Each antenna element has aconnector to enable connection of an antenna element to a variable pathsimulator. The plurality of antenna elements are mounted to at least onestructure so that the antenna elements are distributed in space over asolid angle that covers substantially less than a full sphere. At leastone structure is provided upon which to mount the antenna elements. Apositioner supports a radio frequency, RF, device to be tested withinthe chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention, and theattendant advantages and features thereof, will be more readilyunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a graph of a power angular spread of a single cluster showinga relative power distribution as a function of azimuth angle;

FIG. 2 is a graph of a distribution along two orthogonal directions;

FIG. 3 is a graph for a two-axis cluster at two different points on asurface of a sphere;

FIG. 4 is a graph for a uniform distribution suitable for representing asingle cluster at an arbitrary point on the surface of a sphere;

FIG. 5 is a graph for an elliptically shaped cluster with a uniformvariation in direction around the center of the cluster;

FIG. 6 is an illustration of a partial surface boundary array suitablefor simulating a cluster of reflections in communication with a variablepath simulator and a MIMO tester;

FIG. 7 is an illustration of how a cluster distribution may be mapped tothe partial boundary array;

FIG. 8 is an illustration of a single cluster boundary array with singlepolarized elements along the edge of the cluster;

FIG. 9 is an illustration of a high resolution antenna array;

FIG. 10 is an illustration of an implementation where the dual polarizedelements are oriented to be approximately co-polarized;

FIG. 11 is an illustration for providing a 3-D manipulation of a fixedsingle cluster boundary array about a device under test (DUT);

FIG. 12 is an illustration of mounting of a single cluster boundaryarray on a theta arm to allow elevating the cluster around a DUT on aturntable;

FIG. 13 is an illustration of rotating the single cluster boundary arrayof FIG. 12 about theta;

FIG. 14 is a diagram of an enclosed test chamber having an antenna arrayconstructed in accordance with principles of the present invention; and

FIG. 15 is a flowchart of an exemplary process of simulating a clusterof reflections.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail exemplary embodiments that are in accordancewith the present invention, it is noted that the embodiments resideprimarily in combinations of apparatus components and processing stepsrelated to evaluating radiated performance of MIMO wireless devices inthree dimensions. Accordingly, the system and method components havebeen represented where appropriate by conventional symbols in thedrawings, showing only those specific details that are pertinent tounderstanding the embodiments of the present invention so as not toobscure the disclosure with details that will be readily apparent tothose of ordinary skill in the art having the benefit of the descriptionherein.

As used herein, relational terms, such as “first” and “second,” “top”and “bottom,” and the like, may be used solely to distinguish one entityor element from another entity or element without necessarily requiringor implying any physical or logical relationship or order between suchentities or elements.

In considering the possible continuum of propagation environments that awireless device might encounter, the range varies from the extremelyhigh multipath behavior produced by a reverberation chamber (whichproduces an excellent multi-path environment for MIMO operation, butisn't highly likely to be seen to that extent in any real worldscenario) to a direct line of sight, with no multi-path and no realde-correlation between the transmitted/received signals. The direct lineof sight (LOS) case in a completely anechoic environment is similar tothe test methodologies currently used to evaluate the performance ofSISO (single antenna) radios. However, this approach is not suitable fortesting the performance enhancements of multiple antenna wirelesstechnologies like MIMO and diversity.

At the edge of this continuum near the LOS case is the concept of asingle cluster of reflections, which produces a multipath propagationcondition with only a small amount of differentiation (i.e. the signalsare still highly correlated along the main direction of propagation).Such a cluster of reflections may be encountered in the real world. Forexample, a main reflector may be a broad side of a nearby building withcorrelated reflections from nearby objects. Unlike the uniformlyilluminated and highly uncorrelated environment of a reverberationchamber, where any MIMO device can be expected to perform reasonablywell, the single cluster scenario provides an environment that can beexpected to produce a notably different level of performance between a“good” and “bad” MIMO device due to the difficulty in sorting out thehighly correlated signals from the multiple transmitters. Conversely,the single cluster only provides an indication of the performance of thedevice in a principal direction of the cluster. By manipulating theorientation of the cluster relative to the DUT, it is possible toevaluate the performance of the device as a function of direction.Additionally, the average of that directional performance provides anindication of the average performance of the device that may beconsiderably different from its performance in a statistically uniform(averaged) environment.

Standardized channel models may only define two-dimensional models witha cluster represented by a distribution in azimuth about the DUT asshown in FIG. 1. The distribution of FIG. 1 may be obtained from anarray of antennas in an arc around the RF device. One may envisionproviding a distribution along two orthogonal directions, as shown inFIG. 2. However, simply providing a distribution along two orthogonaldirections does not necessarily produce the desired result. Forspherical coverage, the commonly chosen directions correspond to thetheta and phi directions of a spherical surface; however, thoselocations are not orthogonal at all points on the surface of a sphere,and thus such a distribution becomes distorted at orientations otherthan theta=90°, as shown in FIG. 3. One solution for this is to define acluster using a radial distribution on the surface to produce adistribution such as shown in FIG. 4. This provides a more realisticrepresentation of a “spot” on the surface of a sphere that represents acluster of reflections from a given region. The shape of the spot(cluster) can be varied to represent a realistic geometry, as shown, forexample, in FIG. 5, without being affected by the sphericalrepresentation used in most measurement systems.

FIG. 6 shows an embodiment of an array of antenna elements 52distributed in space over a solid angle having an angular spread thatmay be used to simulate a cluster of reflections about a givenpropagation direction around a device 58. In some embodiments, the solidangle covers substantially less than a full sphere. For example, thesolid angle may be less than 120 degrees, less than 90 degrees, lessthan 60 degrees, less than 45 degrees or less than 30 degrees. Theangular spread of the antenna elements may be one value in one directionand another value in another direction, such as for an ellipticaldistribution of antenna elements. Thus, as used herein, the term ‘solidangle’ may refer to a spherical solid angle or a solid angle of anothershape.

The array of elements 52 may be referred to herein as a boundary arraybecause the elements may be visualized as being on a surface that boundsa device under test. Each linearly polarized element of the array may befed simultaneously by a variable path simulator 57, such as a channelemulator, configured to produce a fading profile with the targetenvironmental specifications including the angular spread of thesimulated cluster in each direction. One result is a directional powerprofile applied to the device with controlled variability inpolarization and orientation such as shown in FIG. 7.

As shown in FIG. 6, the variable path simulator 57 has M paths incommunication with up to M antennas and N paths in communication withanother device 59 or antenna array. In some embodiments, the variablepath simulator 57 is implemented with passive components only, whereasin other embodiments, the variable path simulator 57 includes activecomponents. The variable path simulator 57 may provide communicationbetween the boundary array of FIG. 6, and a MIMO tester 59, a radio suchas a radio base station or wireless device, another boundary array oranother variable path simulator. In some embodiments, the MIMO tester orthe radio may be integrated with the variable path simulator.

The variable path simulator 57 may be used to weight signals from or tothe antenna elements 52 and to introduce multi-path propagation to thesystem. The combination of the spatial distribution of the antennaelements, the orientation of the antenna elements, and weighting appliedby the variable path simulator may cause a simulation of a near fieldenvironment arising from a cluster of reflections of a multipathenvironment. In some embodiments, the variable path simulator transmitssignals to the antenna elements or receives signals from the antennaelements. In some embodiments, the variable path simulator may bothtransmit and receive signals simultaneously or sequentially.

The boundary array method reproduces a target environment by using anarray of independent antenna elements 52 to reproduce the RF conditionsat a bounding surface around the device 58. In the ideal case of aninfinite number of infinitesimally small dual polarized dipole elements,any desired conditions may be re-created within the region enclosedwithin the volume. For the more realistic case of a finite number ofelements, the element spacing on the bounding surface determines the netseparation distance over which two antenna elements within the testvolume will see the same field structure as in the ideal case. Beyondthis maximum correlation distance, the resulting behavior seen by thedevice 58 breaks down and the device 58 is essentially able to resolvethe individual elements in the boundary array. In the case of a singlecluster illumination of the device, only a relatively small portion ofthe boundary surface around the device needs to generate a signal, sincethe contribution from other orientations approaches zero. Thus, the samenumber of channel emulation resources and boundary array antennas thatmight be used to generate an azimuthal or spherical boundary conditionmay be condensed into a much smaller area, increasing the overall sizeof the test volume over which the ideal correlation behavior ismaintained.

Thus, in some embodiments, the antenna array elements may be distributedevenly over a solid angle or may be distributed unevenly with a greaterdensity of elements close to a center of the array and a lower densityof elements away from the center of the array, for example. As anotherexample, the antenna elements may be positioned on an imaginaryspherical or parabolic surface. The spatial distributions of the antennaelements, the orientations of the antenna elements and the excitationsapplied to the antenna elements may be chosen to create a specificdesired radiation distribution such as, for example, a substantiallyGuassian distribution, a substantially Laplacian distribution or asubstantially elliptical distribution.

To further reduce the number of channel emulation resources, possibleimplementations include using only one polarization in each radialorientation of the single cluster array, as shown in FIG. 8. The neteffect of the cluster is still a dual polarized behavior that cansupport an angular distribution and a cross polarization ratio (XPR),but with roughly half the number of antenna elements. In particular,some embodiments may include at least one single polarized element andat least one dual polarized element. Likewise, possible implementationsmay have more antenna elements to produce a higher resolution boundarycondition or support larger angular spreads for a given cluster, asshown in FIG. 9. In addition, the orientations of the antenna elementsmay be have a desired orientation. Certain orientations may make iteasier to calculate the desired field distribution in the channel model.FIG. 10 illustrates an embodiment where all elements are oriented to beapproximately co-polarized, allowing for implementation of a theta/phitype orthogonal coordinate measurement system. Note, however, that auniform surface distribution of the cluster may prevent all elementsfrom being exactly co-polarized, since the orientation of a phi axis cutis not truly orthogonal to a theta cut at all points on the surface of asphere.

The benefits of a cluster of antenna elements to simulate a cluster ofreflections, in terms of boundary array resolution, may only berealized, in some embodiments, in terms of wireless device evaluation ifthe device performance can be evaluated with the same cluster orientedin different directions around the device. For small devices, such asthe device 58, that can be mounted on a positioner 56 and manipulated intwo axes of motion, the embodiment shown in FIG. 11 can be used, wherethe array of antenna elements 52 remains fixed, mounted to the walls orother structure in an anechoic chamber, and the device is rotated on amulti-axis positioning system (MAPS) 56. An alternate embodiment is tomove the cluster of antenna elements 52 on a theta arm 54, as shown inFIG. 12, where the array has been specifically designed to provide thenecessary clearance for the phi axis positioner 56 to allow the centerof the cluster to approach theta=180°, as shown in FIG. 13. Likewise,the phi axis turntable 56 shown in FIG. 12 could be mounted on agoniometer tilt axis to allow it to tilt towards or away from the singlecluster array, again fixed on the wall or ceiling, or with a smallerrange of motion itself.

Thus, the antenna elements can be mounted on one or more structures.Such structure(s) can be rotated about an axis. Such rotation may enablethe structure and antenna elements to be rotated about a device undertest, DUT. For example, as shown in FIGS. 12 and 13, the structure mayinclude spaced apart arms that emanate from a center of the structure.The RF device to be tested may itself be positioned on a positioner thatrotates about one or more axis. An axis of rotation of the positionermay be the same or different from an axis of rotation of the array ofantenna elements mounted on a rotatable structure. Thus, in someembodiments, the array rotates and the device under test rotates so thatthe array may point at any point on the device under test, providingfull spherical coverage. In some embodiments, only the device under testrotates about two axis to provide full spherical coverage.

In some embodiment, a third axis of rotation could be added that isorthogonal to the first two axes of rotation to provide rotation aboutall three Euler angles, thereby allowing for rotation of both thelocation and the polarization of the cluster relative to the deviceunder test. Also, the statistical distribution of the cluster can berotated electronically by the variable path simulator to produce adesired third axis of rotation, changing the shape and polarization ofthe cluster applied to the array.

FIG. 14 shows a test chamber 50 to test a response of an RF device to acluster of reflections of a multipath environment. In some embodiments,the test chamber is an anechoic chamber that may be an enclosed room 50that is partially or fully lined with anechoic absorber (not shown). Theenclosed room 50 has a plurality of antenna elements 52. Each antennaelement 52 has a connector by which the antenna element may be connectedto a variable path simulator (not shown). The variable path simulatormay be located interior of the room or exterior of the room. At leastone structure 54 is provided, upon which the antenna elements 52 aremounted in such a way that the antenna elements are distributed in spaceover a solid angle that is substantially less than a sphere. Apositioner 56 is provided to support an RF device 58 to be tested withinthe chamber. In some embodiments, the structure upon which the antennaelements are mounted is rotatable about a first axis, and the positioneris rotated about at least a second axis. The second axis may be parallelor perpendicular to the first axis.

FIG. 15 is a flowchart of an exemplary process for simulating radiationfrom a cluster of reflections to illuminate a device under test. Aspatial distribution of antenna elements over a solid angle having anangular spread is provided, where the solid angle is substantially lessthan a sphere (block S100). Each antenna element is connected to avariable path simulator, such as a channel emulator, to applyexcitations to the antenna elements (block S102). A cluster ofreflections may be simulated by adjusting at least one of the spatialdistribution, orientations, and excitations of the antenna elements orweights applied to signals from the antenna elements.

Note that additional elements could be added to the array, and theelectrical center of the array could be shifted by suitable excitationof the array elements to simulate a cluster emanating from a differentdirection. This may be done instead of or in addition to rotating thecluster of antenna elements. Note also, that the invention has beendescribed primarily by reference to excitation of the antennas via asimulator. Conversely, the antennas can be excited by radiation from adevice under test to produce at each antenna element a signal that maybe weighted by the simulator to emulate a cluster of reflections.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed herein above. In addition, unless mention was made above tothe contrary, it should be noted that all of the accompanying drawingsare not to scale. A variety of modifications and variations are possiblein light of the above teachings without departing from the scope andspirit of the invention, which is limited only by the following claims.

What is claimed is:
 1. A system for simulating a cluster of reflections,the system comprising: an array of antenna elements distributed in spaceover a solid angle having an angular spread, the solid angle coveringsubstantially less than a full sphere, each antenna element having aspatial orientation; and a variable path simulator in communication withthe antenna elements, the combination of the spatial distribution of theantenna elements, the orientation of the antenna elements, and weightingapplied by the variable path simulator causing a simulation of a nearfield environment arising from a cluster of reflections of a multipathenvironment.
 2. The system of claim 1, wherein the antenna elements aredistributed evenly over the solid angle.
 3. The system of claim 1,wherein the antenna elements are positioned on a spherical surface. 4.The system of claim 1, wherein a density of antenna elements close to acenter of the array is greater than a density of antenna elementsfurther away from the center of the array.
 5. The system of claim 1,wherein the spatial distribution, the spatial orientations andexcitations of the antenna elements are chosen to cause the array toexhibit a radiation distribution that is substantially Guassian.
 6. Thesystem of claim 1, wherein the spatial distribution, the spatialorientations and excitations of the antenna elements are chosen to causethe array to exhibit a radiation distribution that is substantiallyLaplacian.
 7. The system of claim 1, wherein the spatial distribution,the spatial orientations and excitations of the antenna elements arechosen to cause the array to exhibit a radiation distribution that issubstantially elliptical.
 8. The system of claim 1 wherein the weightsapplied by the variable path simulator are chosen to shift an electricalcenter of the array.
 9. The system of claim 1, wherein the antennaelements are mounted on a structure that includes spaced apart arms thatemanate from a center of the structure.
 10. The system of claim 9,wherein the structure is rotatable about an axis to rotate the array ofantenna elements about a device under test.
 11. The system of claim 1,wherein the antenna elements are mounted on a structure that isrotatable about a first axis to rotate the array of antenna elementsabout a device, and the system further includes a device positioner thatis rotatable about at least a second axis to rotate the device.
 12. Thesystem of claim 11, wherein one of the structure and the positioner arefurther rotatable about a third axis different from the first or secondaxis.
 13. The system of claim 1, wherein at least one of the antennaelements is dual-polarized and at least one of the antenna elements issingle-polarized.
 14. A method of simulating a cluster of reflections,the method comprising: providing a spatial distribution of antennaelements over a solid angle having an angular spread, the solid anglecovering substantially less than a full sphere; connecting each antennaelement to a variable path simulator, the variable path simulatorconfigured to communicate with the antenna elements; and adjusting atleast one of the spatial distribution, orientations, and weightingapplied by the variable path simulator to simulate a near fieldenvironment arising from a cluster of reflections.
 15. The method ofclaim 14, further providing an anechoic chamber surrounding the spatialdistribution of antenna elements.
 16. The method of claim 14, furthercomprising providing the spatial distribution of antenna elements on astructure that is rotatable about a device under test.
 17. An anechoictest chamber to test a response of an RF device to a cluster ofreflections of a multipath environment, the anechoic test chambercomprising: an enclosed shielded room at least partially lined with RFabsorber; a plurality of antenna elements, each antenna element having aconnector to enable connection of an antenna element to a variable pathsimulator, the plurality of antenna elements mounted to at least onestructure so that the antenna elements are distributed in space over asolid angle that covers substantially less than a full sphere; and atleast one structure upon which to mount antenna elements; and apositioner to support a radio frequency, RF, device to be tested withinthe chamber.
 18. The anechoic test chamber of claim 17, wherein the atleast one structure is rotatable about a first axis.
 19. The anechoictest chamber of claim 18, wherein the positioner is rotatable about asecond axis.
 20. The anechoic test chamber of claim 19, wherein thepositioner is further rotatable about a third axis.